How to grow a Moss Wall

Get a plastic mesh. It's available at hardware shops or plant nurseries. The holes should be about 8 mm in diameter.

Cut out the size you want. If you are planning on a backdrop and your tank's dimensions are 4 ft (Length) and 2 ft (Height), then you should cut out a piece that is 8 ft X 2 ft.

Fold the mesh into 2. Along the edges and at random around the middle, push in suction caps through the holes of the mesh.
Unfold the mesh and spread out the moss on only one side. Try to spread the moss as evenly as possible. If you leave a big space without any moss, chances are that there will be a gap in your moss wall later on.
Fold the mesh into 2 again and tie up the edges. It's better not to use string. Use something that won't eventually dissolve or melt after being soaked in water for a long time.
Position your mesh against the glass at the back of your tank. It's important that the edges of the mesh are stuck close to the glass of the tank. Do not leave gaps between the glass and the mesh as they are death traps for fishes.
Now all you have to do is wait for the moss to grow out from the holes in the mesh to form a beautiful moss wall. Piece of cake, really :)